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<h2 id="Summary" name="Summary">概述</h2>
<p><strong><code>toExponential()</code></strong> 方法以指数表示法返回该数值字符串表示形式。</p>
<h2 id="Syntax" name="Syntax">语法</h2>
<pre><code class="language-javascript"><code><em>numObj</em>.toExponential(<em>fractionDigits</em>)</code></code></pre>
<h3 id="Parameter" name="Parameter">参数</h3>
<dl>
<dt>fractionDigits</dt>
<dd>可选。一个整数，用来指定小数点后有几位数字。默认情况下用尽可能多的位数来显示数字。</dd>
</dl>
<h3 id="Returns" name="Returns">返回值</h3>
<p>一个用幂的形式 (科学记数法) 来表示<a href="Reference/Global_Objects/Number" title="JavaScript 的 Number 对象是经过封装的能让你处理数字值的对象。Number 对象由 Number() 构造器创建。"><code>Number</code></a> 对象的字符串。小数点后以fractionDigits 提供的值来四舍五入。如果 <code>fractionDigits</code> 参数被忽略了，小数点后的将尽可能用最多的位数来表示该数值。</p>
<p>对数值字面量使用 <code>toExponential()</code> 方法，且该数值没有小数点和指数时，应该在该数值与该方法之间隔开一个空格，以避免点号被解释为一个小数点。也可以使用两个点号调用该方法。</p>
<p>如果一个数值的小数位数多余 <code>fractionDigits</code> 参数所提供的，则该数值将会在 <code>fractionDigits</code> 指定的小数位数处四舍五入。可以查看 <a href="Reference/Global_Objects/Number/toFixed" title="toFixed() 方法使用定点表示法来格式化一个数。"><code>toFixed()</code></a> 方法描述中关于四舍五入的讨论，同样应用于 <code>toExponential() </code>方法。</p>
<h3 id="Throws" name="Throws">异常</h3>
<dl>
<dt><a href="Reference/Global_Objects/RangeError" title="RangeError对象标明一个错误，当一个值不在其所允许的范围或者集合中。"><code>RangeError</code></a></dt>
<dd>如果 <em>fractionDigits</em> 太小或太大将会抛出该错误。介于 0 和 20（包括20）之间的值不会引起 <code>RangeError</code> 。 执行环境也可以支持更大或更小范围。</dd>
</dl>
<dl>
<dt><a href="Reference/Global_Objects/TypeError" title="TypeError（类型错误） 对象用来表示值的类型非预期类型时发生的错误。"><code>TypeError</code></a></dt>
<dd>如果该方法在一个非数值类型对象上调用。</dd>
</dl>
<h2 id="Example" name="Example">示例</h2>
<pre><code class="language-js">var numObj = 77.1234;

alert("numObj.toExponential() is " + numObj.toExponential()); //输出 7.71234e+1

alert("numObj.toExponential(4) is " + numObj.toExponential(4)); //输出 7.7123e+1

alert("numObj.toExponential(2) is " + numObj.toExponential(2)); //输出 7.71e+1

alert("77.1234.toExponential() is " + 77.1234.toExponential()); //输出 7.71234e+1

alert("77 .toExponential() is " + 77 .toExponential()); //输出 7.7e+1</code></pre>
<h2 id="规范">规范</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">规范版本</th>
<th scope="col">规范状态</th>
<th scope="col">注解</th>
</tr>
<tr>
<td>ECMAScript 3rd Edition. Implemented in JavaScript 1.5</td>
<td>Standard</td>
<td>Initial definition.</td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-15.7.4.6" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">Number.prototype.toExponential</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-number.prototype.toexponential" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Number.prototype.toExponential</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<p></p><div class="blockIndicator warning"><strong><a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">We're converting our compatibility data into a machine-readable JSON format</a></strong>.
            This compatibility table still uses the old format,
            because we haven't yet converted the data it contains.
            <strong><a class="new" href="/zh-CN/docs/MDN/Contribute/Structures/Compatibility_tables" rel="nofollow">Find out how you can help!</a></strong></div>
<div class="htab">
<a id="AutoCompatibilityTable" name="AutoCompatibilityTable"></a>
<ul>
<li class="selected"><a>Desktop</a></li>
<li><a>Mobile</a></li>
</ul>
</div><p></p>
<div id="compat-desktop">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Chrome</th>
<th>Firefox (Gecko)</th>
<th>Internet Explorer</th>
<th>Opera</th>
<th>Safari</th>
</tr>
<tr>
<td>Basic support</td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
</tr>
</tbody>
</table>
</div>
<div id="compat-mobile">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Android</th>
<th>Chrome for Android</th>
<th>Firefox Mobile (Gecko)</th>
<th>IE Mobile</th>
<th>Opera Mobile</th>
<th>Safari Mobile</th>
</tr>
<tr>
<td>Basic support</td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
</tr>
</tbody>
</table>
</div>
<h2 id="See_Also" name="See_Also">相关链接</h2>
<ul>
<li><a href="Reference/Global_Objects/Number/toFixed" title="toFixed() 方法使用定点表示法来格式化一个数。"><code>Number.prototype.toFixed()</code></a></li>
<li><a href="Reference/Global_Objects/Number/toPrecision" title="toPrecision() 方法以指定的精度返回该数值对象的字符串表示。"><code>Number.prototype.toPrecision()</code></a></li>
<li><a href="Reference/Global_Objects/Number/toString" title="toString() 方法返回指定 Number 对象的字符串表示形式。"><code>Number.prototype.toString()</code></a></li>
</ul>
</article>